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sulphuric acid in presence of methyl orange.
gram HgCl2.

i c.c. N-NaOH = 0-1084


Hg2Cl2 = 470-9

A white powder, insoluble in water, alcohol or ether. It is sold as :
Sublimed calomel, transparent crystals under the microscope; calomel
condensed in steam, particles which are opaque or at most transparent at
the edges ; Precipitated calomel, formed of very minute, amorphous opaque
particles (when the salt is finely triturated, these microscopic characters
are no longer discernible). As impurities or adulterants there may appear
mercuric chloride, mercuric aminochloride (white precipitate), sodium
chloride, barium sulphate, kaolin, lead carbonate, gypsum, chalk, etc.

1.  Fixed Substances.—I gram, heated in a porcelain crucible, should
yield no appreciable residue if pure.

2.  Mercuric Chloride.—-I gram is shaken with 10 c.c. of water and
filtered, the filtrate being tested with silver nitrate and with hydrogen
sulphide ;   with the former scarcely any milkiness and with the latter a
faint brown ccloration are allowable.

3.  White Precipitate.-—The sample is heated with excess of caustic
potash solution;   in presence of white precipitate, ammonia is evolved.


The chromium compounds used in dyeing and tanning include, besides
chromic acid and sodium and potassium chromates and dichromates (see
separate articles), the following :

Chromium acetate, either neutral, Cr(C2H3Oa)3, or basic, CrOH(C2H302)2,
in green or violet solution of 20°, 24° and 30° Baume or in the dry state.
Chromium Sulphoacetate and Nitroacetate, which are mixtiires of the acetate
with the sulphate or nitrate of chromium in various proportions.

Chromium chloride, in solution of 20-30° Baume, usually containing
basic chlorides, CrCl(OH)2, CrCl2(OH) and Cr2C]3(OH)3, and often con-
taminated with alkali salts, sulphates and iron.

Chromic fluoride, CrF3 + 4H20, in green crystals or powder.

Chromic formate, Cr(HC02)3 or CrOH(HC02)2, in green solution.

Chromic hydroxide, Cr(OH)3, in greyish-green paste or powder.

Chromic sulphate, Cr2(S04)3 + I5H20 ; basic sulphates of various com-
positions ; chromium and potassium sulphate or chrome alum, Cr2(S04)3,
K2S04 + 24H20, in dark reddish-violet crystals ; mixtures in various
proportions of more or less basic chromium sulphate with sodium sulphate
(often also with the formate) either in green solution or in small green,
apparently crystalline fragments. All these sulphates may be contami-
nated with gypsum, free sulphuric acid and tarry matters.

Analysis of these products includes investigation of the bases and acidse, and exactly 10 c.c. of N-sodimn hydroxide ; the liquid is shaken,